MIDI Parts in folders don’t inherit their colors from the Track colors properly.
A MIDI Track without any color assigned to it and outside a folder inherits properly the color of the MID Track family which you can set in preferences
A MIDI Track without any color assigned to it and inside a folder gets a generic color of some kind
When you move the Track outside the Folder the part changes color
This works for Audio Parts too.
Not the end of the world as we know it, buuuut …